1. Field of the Invention
This invention is concerned with an improved method and combination process for converting synthesis gas, i.e. mixtures of gaseous carbon oxides with hydrogen generally CO rich, to hydrocarbon mixtures.
Processes for the conversion of coal and other hydrocarbons such as natural gas to a gaseous mixture consisting essentially of hydrogen and carbon monoxide, or of hydrogen and carbon dioxide, or of hydrogen and carbon monoxide and carbon dioxide, are well known. An excellent summary of the art of gas manufacture, including synthesis gas, from solid and liquid fuels, is given in Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 10, pages 353-433 (1966), Interscience Publishers, New York, N.Y., the contents of which are herein incorporated by reference. The techniques for gasification of coal or other solid, liquid or gaseous fuel are not considered to be a part of this invention.
It is well known that synthesis gas comprising carbon monoxide and hydrogen will undergo conversion to form reduction products of carbon monoxide, at temperatures in the range of 300.degree. F. to about 850.degree. F. and pressures in the range of one to one thousand atmospheres, over a fairly wide variety of catalysts. The Fischer-Tropsch process, for example, which has been extensively studied, produces a range of hydrocarbons, waxy materials and some liquid materials which have been used as low octane gasoline. The types of catalysts that have been studied for this and related processes include those based on metals or oxides or iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium.
The range of catalysts and catalyst modifications disclosed in the art encompasses an equally wide range of conversion conditions for the reduction of carbon monoxide by hydrogen and provides considerable flexibility toward obtaining selected boiling range products. Nonetheless, in spite of this flexibility, it has not been possible heretofore to provide a catalyst for medium pressure operation (5-30 atm) which will produce particularly olefin compositions comprising primarily internal double bond characteristics and boiling in the gasoline boiling range. A review of the status of this art is given in "Carbon Monoxide-Hydrogen Reactions", Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 4, pages 446-488, Interscience Publishers, New York, N.Y.
The conversion of synthesis gas to hydrocarbon mixtures is disclosed in copending application Ser. No. 583,353, filed June 2, 1975, and copending application Ser. No. 566,167, filed Apr. 4, 1975. Compositions of iron, cobalt or nickel deposited in the inner absorption regions of crystallize zeolites are described in U.S. Pat. No. 3,013,990. Attempts to convert synthesis gas over X-zeolite base exchanged with iron, cobalt and nickel are described in Erdoel und Kohle--Erdgas, Petrochemie; Brennstoff--Chemie, Volume 25, No. 4, pages 187-188, April 1972.
One particularly desirable catalyst used in the conversion of syngas has been potassium promoted iron, which has been used in combination with special types of zeolites, such as ZSM-5, in order to produce valuable hydrocarbons. Thus, for example, copending Application Ser. No. 566,167, now abandoned, is directed towards the conversion of syngas with potassium promoted iron in admixture with HZSM-5. Although the process of this copending application is indeed effective in producing products having a substantial quantity of aromatics, nevertheless there are disadvantages associated with said process, primarily in the regeneration aspect of the catalyst. It is known that when processes of this type are operated under conditions which favor the production of aromatics, there are also produced substantial amounts of coke which are deposited about the acid ZSM-5 catalyst. This requires that the catalyst be subjected to frequent regeneration, and due to the fact that the process of said copending application Ser. No. 566,167 involved a catalyst mixture containing an iron catalyst and a ZSM-5 catalyst, the extent and amount of regeneration were limited by the effect that the regeneration would have on the iron component. Thus, although HZSM-5 by itself exhibits a remarkable stability with regard to regeneration of the same by burning off carbon deposits, the same is not true with respect to a Fischer-Tropsch catalyst in general, and iron promoted potassium in particular.
In U.S. Pat. No. 4,086,262 issued Apr. 25, 1978, there is disclosed a process for the conversion of synthesis gas using a single stage process wherein the catalyst is a mixture of an iron containing Fischer-Tropsch catalyst and a ZSM-5 type zeolite. One of the examples, however, is directed towards a two bed operation wherein syngas is contacted over a first fixed catalyst bed containing an iron catalyst and the total product is thereafter contacted in a second fixed catalyst bed containing a ZSM-5 type zeolite. The example resulted in poor aromatic production and excessive methane production.
In copending Application Ser. No. 614,586, filed Sept. 18, 1975, now U.S. Pat. No. 4,046,830, issued Sept. 6, 1977, there is disclosed a process wherein the total effluent from a Fischer-Tropsch operation is upgraded over a ZSM-5 type zeolite. Although the process of said application is indeed a valuable one, it has been found that the process can be improved by operating within a more selective range of process conditions. Thus, the instant invention represents an improvement over the operation of U.S. Pat. No. 4,046,830 and copending Application Ser. No. 826,487 filed Aug. 22, 1977 and now U.S. Pat. No. 4,159,995.